The invention relates to a coating apparatus and, more particularly, to an improved rotary atomizer coater for depositing paint or other fluid coatings on a workpiece. The rotary atomizer coater of the present invention may be used with a robot mounting or in connection with other types of work place applications or apparatus.
Rotary atomizer coaters having a disk or bell cup are known in the art. Often the bell cups are driven by air motors, such as air driven turbines. Examples of prior art rotary atomizer coaters are described in U.S. Pat. Nos. 4,405,086 and 4,555,058. In rotary atomizer coaters a fluid coating which is to be atomized is supplied and charged electrostatically with a high voltage. Often these voltages are from approximately 20 to 100 KV. The bell cups are rotated at high speeds normally between 10,000 and 40,000 rpm. The charged fluid is fed to an inner chamber of the rotating bell cup where it is centrifuged forwardly through chamber openings to the large diameter edge of the cone and is broken up into atomized particles as it escapes the bell cup edge. The axial centerline of the spinning bell cup points toward an electrically grounded workpiece which is to be coated. Normally, the workpiece is located approximately 10 inches (25.4 cm) in front of the bell cup. Because the atomized particles are centrifuged in a direction perpendicular to the axial centerline of the bell, it is known in prior art devices to redirect these particles so that they move toward the workpiece. In some situations, the electrostatic charge held by the atomized coating particles is sufficient to attract the particles to the grounded workpiece. It is also known in prior art rotary coaters to supply a cylindrically shaped curtain of shaping air which also directs the particles toward the workpiece. The shaping air also controls the diameter of the spray particle pattern. Prior art atomizer coaters having shaping air apparatus are described in U.S. Pat. Nos. 4,776,520 and 4,899,936. In many prior art rotary atomizer coaters, when a coating fluid is delivered to the bell cup in excess of the amount which can be dispensed by the bell cup, the coating fluid often will backup into the shaft and housing openings at the rear of the bell cup. This is undesirable, particularly when such coating fluid can damage the rotary mechanism of the shaft and bearing assembly.
In any electrically charged rotary atomizer system, it is important to make every effort to minimize the chance of an arc of sufficient energy which can ignite the coating fluid being fed to or being atomized by the bell cup.
In prior art atomizer coaters, where a cylinder of shaping air is utilized to direct the particles as they leave the bell cup edge, the high velocity shaping air often creates a negative pressure zone and causes ambient air to be drawn towards and along side the shaping air. Electrically charged particles in the vicinity are often drawn backwardly into this negative pressure zone, where they tend to become deposited on the surface of the rotary atomizer coater housing.
High voltage ladder or cascade power supplies are known in the art. It has been found that because the electronic components in these potted or encapsulated power supplies create heat which cannot be easily removed, many of the prior art power supplies are short lived.
Another problem found in the prior art are found in the bearings which rotatably mount the rotary atomizer coater shaft. Air bearings are known in prior art rotary atomizer coaters but are generally complex and expensive. These prior art air bearings often use composite materials including bronze, white metal and other materials which are mixed, formed to a close tolerance and machined. A tetrafluoroethene homopolymer, such as a TEFLON material is often mixed with the bearing material to reduce friction.
Because bell cups are rotated at high speeds, it is important to accurately mount the bell cups on the shaft to minimize out-of-balance radial load amplification at high speeds.